This invention relates to production of a reducing gas useful in a direct reduction process and, more particularly, this invention relates to the use of a high-temperature gas-cooled (HTGR) nuclear reactor coupled with a reformer and a cryogenic gas separation unit for the production of a reducing gas for use in the direct reduction process.
In view of the world's serious energy problems and, particularly, the world shortage of metallurgical grade coking coal that is required to support the traditional blast furnace processes of reducing iron ore, it would be advantageous to have some method of meeting the world's steel requirements, other than conventional methods, that would conserve the shortage of raw materials and, particularly, fossil fuels.
One such method involves the use of nuclear energy. Probably the most likely possibilities for the application of nuclear energy to steelmaking are two well known processes--direct reduction in a shaft furnace and refining in an electric furnace. Although direct reduction is a fairly new process, it is becoming well developed and is in commercial use in various parts of the world where low-cost natural gas is available. In the direct reduction process, lump iron ore or iron ore pellets are reduced in the solid condition to a product sometimes known as sponge iron, containing less oxygen than ore. The reaction requires high temperatures and is also highly endothermic, requiring substantial amounts of heat. Nuclear energy could be used to provide at least a portion of the heat required for direct reduction of iron ore. Processes for direct reduction differ in certain details; however, almost all these processes use as a redundant a gas mixture of carbon monoxide and hydrogen at temperatures from about 1400.degree. to about 1800.degree. F.
In conventional direct reduction plants using fossil-fired reformers, it is customary to burn the off-gas from the direct reduction shaft furnace as fuel for the reformer. In a nuclear heated reformer, there is no need for fossil-firing so that a shaft furnace off-gas may be purified and recirculated to realize a desirable fuel efficiency. Therefore, natural gas requirements can be reduced from about 13,000 scf per ton of reduced product for conventional reforming to from about 5,500 to about 8,000 scf per ton of reduced product for nuclear reforming. Even in the most inefficient case I studied, in using my invention, a natural gas savings of about 35 percent was realized; and in the most efficient case, the savings approached 60 percent. This principle of recirculation imposes stricter requirements on the level of contaminating inert gases and unreacted hydrocarbons because they are built up to unacceptable levels in the recirculating loop. For this reason, it is necessary to provide a method of removing excess water and unreacted hydrocarbons from the reformer exit gases before these gases can be considered acceptable for a direct reduction process.
The production of steel by electric arc furnaces is a long established commercial process. Electric furnace capacity in the United States alone is about 30 million tons a year. Almost all of that tonnage is made with scrap as the only ferrous charge. Sponge iron could be used for a large portion of the charge if the cost were competitive with scrap.
I am aware of current development work being conducted in Europe to provide a method of producing an acceptable reducing gas with the use of nuclear power. However, the work is centered around the development of a new generation of HTGR nuclear reactors capable of attaining core coolant temperatures several hundred degrees higher than are obtained in present day HTGR nuclear reactors. Core coolant temperatures of about 1750.degree. F are required in order to obtain high enough reforming temperatures to minimize methane breakthrough to an acceptable level.
The Japanese are also developing another new concept of an HTGR nuclear reactor capable of obtaining core coolant temperatures of about 1830.degree. F. Both of the HTGR nuclear reactor concepts, however, require considerable development work and testing before they can be considered acceptable for commercial operation. Such a develoment will require a substantial expenditure of time and money, extending the time when nuclear steel could be commercial.
I am also aware of the use of a cryogenic separation unit for use in ammonia plants as taught in an article published in HYDROCARBON PROCESSING, entitled "Syngas Purifier Cuts Ammonia Costs," by Bernard J. Grotz, C. F. Braun and Co., Alhambra, Calif.
I have invented a method which will greatly expedite the time necessary to commercialize nuclear steelmaking by employing a prismatic design HTGR nuclear reactor, such as manufactured by General Atomic Company of San Diego, Calif., and presently installed in a power reactor at Ft. St. Vrain, Col., which operates with a coolant pressure of about 700 psig, which has been found to be optimal for efficient reactor operation. For mechanical reasons, it is desirable to design any reformer or heat exchanger using reactor coolant as a heating medium at a pressure of about 500 psig to minimize the pressure differential across the tubes and tube sheets. At this temperature and pressure an unacceptable amount of methane breakthrough will be experienced in the reforming exit gases. Therefore, I provide an HTGR nuclear reactor as a heat source for a catalytic reformer which is coupled with a cryogenic gas separation unit to remove a substantial amount of the unreacted hydrocarbon, thereby rendering the reformer gas acceptable for a direct reduction process.
I am also aware of the following prior art:
______________________________________ Patent No. Date Inventor U. S. Classification ______________________________________ 2,998,303 8/61 Huebler 23-212 3,026,683 3/62 Palazzo, et. al. 62-17 3,136,623 6/64 Mader, et. al. 75-34 3,148,050 9/64 Bogdandy 75-34 3,282,677 11/66 Futakuchi, et. al. 75-34 3,315,475 4/67 Harmens 62-12 3,382,045 5/68 Habermehl, et. al. 23-213 3,453,835 7/69 Hochgesand 62-17 3,532,467 10/70 Smith, et. al. 23-212 3,591,364 7/71 Reynolds, et. al. 75-42 3,594,305 7/71 Kirk, Jr. 208-10 3,618,331 11/71 Smith, et. al. 62-11 3,628,340 12/71 Meisler 62-18 ______________________________________